It is the aim of the study to chracterize further the common enterobacterial antigen and its biologic significance, particularly with regard to its possible use as an immunogen for protection against infection by gram-negative bacteria sharing the antigen. The genetic control of CA production and the immunogenicity of the CA-producing strains will be studied further using numerous mutants. The immunosuppressive effects of lipopolysaccharides isolated from CA- producing and CA-defective mutants will be compared. The inhibitory effects of serum from various animal species on attachment of lipopolysaccharides to erythroctes will be analyzed. Oral immunization with selected immunogenic strains and mutants of enterobacteriaceae will be carried out in the rabbit, and the serum and local immune response measured by hemagglutination, indirect FA technique, and gel precipitation.